I respect the medical pro standpoint, but from a fighters standpoint: If my hands and feet are wrapped in my opponent's guard, I have one weapon left. The head can be trained just like a hand and foot, if used by an amateur they can hurt themselves badly, but if you know what you're doing then you can use it properly.

Proper headbutt form: Use the crown of the head to strike the face of your opponent. That way you won't damage the soft spots on your face or the base of your spine.